On the necessity of composition-dependent low-temperature opacity in models of metal-poor asymptotic giant branch stars

Abstract

The vital importance of composition-dependent low-temperature opacity in low-mass (M ≤ 3 M⊙) asymptotic giant branch (AGB) stellar models of metallicity Z ≥ 0.001 has recently been demonstrated. Its significance to more metal-poor, intermediate-mass (M ≥ 2.5 M ⊙) models has yet to be investigated. We show that its inclusion in lower-metallicity models ([Fe/H] ≤-2) is essential and that there exists no threshold metallicity below which composition-dependent molecular opacity may be neglected. We find it to be crucial in all intermediate-mass models investigated ([Fe/H] ≤-2 and 2.5 ≤ M/M⊙ ≤ 5), because of the evolution of the surface chemistry, including the orders of magnitude increase in the abundance of molecule-forming species. Its effect on these models mirrors that previously reported for higher-metallicity models - increase in radius, decrease in Teff, faster mass loss, shorter thermally pulsing AGB lifetime, reduced enrichment in third dredge-up products (by a factor of 3-10), and an increase in the mass limit for hot bottom burning. We show that the evolution of low-metallicity models with composition-dependent low-temperature opacity is relatively independent of initial metal abundance because its contribution to the opacity is far outweighed by changes resulting from dredge-up. Our results imply a significant reduction in the expected number of nitrogen-enhanced metal-poor stars, which may help explain their observed paucity. We note that these findings are partially a product of the macrophysics adopted in our models, in particular, the Vassiliadis & Wood mass loss rate which is strongly dependent on radius. © 2014. The American Astronomical Society. All rights reserved.